This invention relates to a method of treating hot rolled nitrogenized low carbon steel which produces a material having a ductility during forming and a yield strength thereafter comparable to that of the commercial hot rolled 80,000 psi yield strength low alloy (HSLA) steels.
The need to reduce the weight of the automobile has become increasingly urgent in recent years with the need to otherwise accommodate weight increases due to additions of safety and emission control devices and to improve engine performance and fuel economy. These considerations have prompted interest in automobile structural materials having a higher strength-to-weight ratio.
One group of such materials presently being considered is the family of the aforementioned high strength low alloy steels with yield strengths in the neighborhood of 80,000 psi. These steels offer an attractive combination of increased strength and acceptable formability.
The high yield strength of the HSLA steel is developed through a controlled combination of grain refinement, precipitation hardening, and solid solution strengthening which results from the addition of titanium, vanadium or niobium to the basic low carbon steel chemistry, and from the carefully controlled cooling in the hot strip mill in which such steels are produced. Often rare earth alloying elements are added to control the shape of the inclusions and hence to improve the steel's formability. Present steel mill processing limitations require hot rolled steels to be used for stampings having thicknesses of 0.070 inch or more. Many automobile components require thicknesses of 0.08 inch or more such as bumper reinforcements and frame components which must be made of hot rolled steel.
The term "low carbon steel" as used herein is a steel containing up to 0.25 percent carbon and only residual amounts of elements other than those required for deoxidation, particularly silicon 0.6 percent or less, and manganese 1.65 percent or less. The term "nitrogenized steel" as used herein is a "low carbon steel" containing nitrogen preferably in the range of 80 to 200 parts per million or 0.008 to 0.02 percent by weight.